Electric hot plate



June 22, 1937'. J D MORGAN ET 2,084,819

ELECTRIC HOT PLATE Filed July 17, 1936 INVENTORS JOHN UMORGAN BY M WATTORNEY Patented June 22, 193? austere unuc'rnie nor rm'ru lohn l).Morgan, Southvllmange, N. L, and Russell E. lLcwe, New York, N. Yassignors to llower Patents Company, llersey (City, N. 45., a corpora=tion of Maine Application July it, 1936,, Serial No. 91,1136

2 Claims,

'lhis invention relates to an electric hot plate, and is moreparticularly concerned with electric heating, units of the closed tophaving refractory resistor-supporting members employed for heatingcooking vessels and the like.

Electric hot plates of the closed top refractory resistor-supportmg typehave long been known. The primary object of the present invention is toprovide a refractory resistor-supporting closed 10 top hot plate whichshall be economical to build and which shall at the same time be moreemcient and possess a longer life than hot plates previously known.

A feature of the invention resides in the use of chromium-iron-aluminumalloy wire as the resistor element. Such chromium-iron-aluminum wirehas, been found to have a working life athigh temperatures which is verymuch longer than that of previously known metal and alloy resistancewire under comparable conditions. it

has been found, however,"that this new chromium-iron-aluminum alloy wirerapidly dieteticrates and its eificiency as an electric resistor wiredrops, when it is embedded in refractories containing alumina and silicaof the type commonly used in constructing refractory resistor-supportingclosed top hot plates. This deterioration of the chrome-iron-aluminumwire in the presence of alumina and silica refractory is apparently theresult of a reducing action of the wire on the oxide refractory at thehigh operating temperatures at which the hot plate is used.

Another feature of the invention consists in embedding thechrome-=iron-aluminum wire in a refractory resistor-supporting membercomprising a phosphoric acid bonded zircon ferro-siliconaluminum hydraterefractory of the type de- -scribed in our copendingapplication S. N.87,662 filed June 27, 1936 for High temperature refracrtory. Thisfeature of the invention is based on the discovery that azirconform-silicon refractory oi? the type described in our aforementionedapplication does not chemically react with chrome-iron-aluminum wire attemperatures as '45 high as 2000-2300" F. and can therefore be used asthe refractory resistor-supporting member for .such wire in constructingclosed top'hot plates which are very eflicient and which have a life 50many times that of hot plates now on the market.

With the foregoingand other objects and feetures in view, the inventionconsists in the improved electric heater and process of making same ashereinafter described and more particularly 55 defined by theaccompanying claims.

(Clo 219 87) In the following description reference will be .rnade tothe accompanying drawing in which Fig. i is a plan view, with parts cutaway, showing the top of an electric hot plate con structed in.accordance with the present invention and illustrating the manner ofdisposing the resistor coil of wire on. the porcelain plate base memberof the heating unit; and

Fig. 2 is a view in vertical side section showing the relativedisposition of the porcelain plate, the 10 coil resistor filament, andthe refractory worksupporting surface of the plate imbedding theresistor filament. v

in the following description the electric hot' plate forming the subjectof the present inven- 15 tion will be described chiefly by the method ofmanufacture. he shown in the drawing, the base member of the hot plateunit consists of a thin porcelain plate Y til (not substantially greaterthan inch maximum thickness), said plate it preferably having a raisedperipheral rim pcr- 20 tion'. 82 and an axial bore M bounded by a raisedcentral rim it. Two small vertical apertures it are provided inthebottom of plate it) at adjacent radially and circumierentially spacedpoints near the rim it for the purpose of receiving the e terminalclamps or other connecting links between the two ends of a metalresistor heating coil it and an outside source of current (not shown)for supplying electrical energy for operating the hot plate. Theresistor 20 consists of 30 a chrome-iron-aluminum alloy wire constructedof 30-elo% chromium, 58% aluminum and iron, together with-in somecases-a small amount (up to 5%) of cobalt. The hot plate in the presentinvention is made up from a helical coil of 35 the alloy wire which islaid out in two concentric spirals which will fit within the rims i2 andit of plate ill, and which coil is annealed to lay flat on the plate illwith its two ends 22 either attached to terminal posts mounted in theapertures M or carried out through the apertures as shown in thedrawing.

As previously explained, the refractory worksupporting member 24 of thehot plate illustrated consists of a special type of refractory havingproperties which do not adversely affect the chemical composition andelectric conductivity of the chrome-iron-aluminum wire 20 whenassociated therewith in intimate electrically uninsulated and heatconducting relation. This refractory is prepared substantially asfollows:

' Native zircon (zirconium silicate) is milled so that most of themilled material will pass through a 300 mesh screen. Ferrosilicon(preferably 75% or 85% silicon) is ground to pass through an 80 meshscreen and be retained on a 300 mesh screen. Forty to fifty parts byweight of the milled grain zircon is wetted with water to a creamyconsistency, and 8 parts by weight of 85% orthophosphoric acid is addedto the wetted zircon. This mixture is digested for a period of about 12hours at a temperature of 175-200 F. The digested zircon-phosphoric acidmixture is then well stirred and further diluted with water to whatmight be termed a creamy consistencyl One part by weight of dry aluminumhydrate is mixed with parts by weight of the milled ferrosilicon and thedry-mixed ferrosilicon and aluminum hydrate is added to the creamypartially digested zircon-phosphoric acid mixture. The resulting mixtureis thoroughly agitated for a brief period and is then allowed to digestfor about 12 hours at a temperature of PIS-200 F. The latter period ofdigesting may be continued until such time as the digested mixture hasdried to a solid condition, but preferably the final drying operation isconducted under a strong vacuum (20 inches or more) and thoroughlyevacuated until all signs of gas ebullition from the mixture haveceased. This solid mixture is then ground and milled to pass through anmesh screen, and is wetted with water to rehydrate the mixture to amolding or pouring consistency (about the consistency of cream).

Because the bonding agent for the refractory member 24 is phosphoricacid, some of which may not have completely reacted in the operation ofpreparing the refractory which has just been described, the followingmethod of forming the member 24 has been adopted. The plate In is firstlaid or mounted on a rubber base or cushion, and the fluid zirconferrosilicon aluminum hydrate refractory is then fed to the top of theplate ID from a rubber container through a rubber hose. The fluidrefractory is poured in two stages, the first stage being to about thelevel or height 26 to which the wire 20 extends above the top surface ofthe plate It! (i. e. to approximately the outside diameter of thehelical coil into which the wire is formed before placing it on theplate). The layer of refractory poured in the first stage is allowed toair set for a few minutes and then a second layer of refractory ispoured to a level with the top of the rims l2 and IS. The heater is thenallowed to set and air dry for several hours, after which it is placedin a bake oven where it is heatl suitable color may be' applied in wellknown manner as by spraying, and the glaze is then dried and set bygradual heating to a temperature above its. softening point, followed bygradual cooling.

It will be understood that while the invention is not limited to theexact compositions of chrome-iron-aluminum alloy resistor wire and ofembedding zircon-ferrosilicon-aluminum hydrate refractory which havebeen stated in the example given, the invention resides particularly inthe combination of alloy resistor wire and embedding refractory of thegeneral type described whereby the life of a hot plate at hightemperatures can be many times increased, and accordingly variations ofsubstantially over plus or minus 5% in the composition of the alloy wireand in the composition of the refractory should not be made. 1

The invention having been thus described, what is claimed as new is:

1. An electric heater comprising a thin porcelain base plate of low heatconductivity, a metallic resistor consisting of chrome-iron-aluminumwire wound in a helical coil and disposed on top of the plate with endsconnected to closely spaced terminal elements, and a refractoryworksupporting plate of high heat conductivity surmounting and bonded tothe porcelain plate and embedding the resistor, said plate consistingessentially of a mixture of milled grain zircon, ferrosilicon and asmall amount (about 1%) of aluminum hydrate bonded with the reactionproducts of phosphoric acid therewith.

2. An electric heater as described in claim 1, in which the alloy wirecontains 30-40% by weight of chromium and 5-8% aluminum, and in whichthe refractory supporting plate contains 40-50 parts by weight of zirconand 50-60 parts by weight of 75% ferrosilicon bonded with about 6 partsby weight of ortho-phosphoric acid.

JOHN D. MORGAN. RUSSELL E. LOWE.

